Various implantable medical devices have been developed that receive information from one or more physiologic sensors or transducers. One such physiologic transducer is a pressure sensor that transduces blood pressure into corresponding electrical signals. The electrical signals produced by the pressure sensor are acquired by an implantable medical device via a lead coupled therebetween.
The sensitivity of a body implantable pressure sensor is an important design characteristic. The sensitivity characteristics of an implantable pressure sensor have a direct impact on the signal-to-noise ratio of the pressure sensor signal transmitted to the implantable medical device via the lead coupled therebetween. Increasing the strength of the signal produced by the implantable pressure sensor would therefore appear desirable, as such an increase would result in improved signal-to-noise characteristics and increased integrity of pressure data transmission.
Efforts have been underway for many years to develop implantable pressure transducers and sensors for temporary or chronic use in a body organ or vessel. Many different designs and operating systems have been proposed and placed into temporary or chronic use with patients. Indwelling pressure sensors for temporary use of a few days or weeks are available, and many designs of chronically or permanently implantable pressure sensors have been placed in clinical use.
Many indwelling pressure sensors employ a piezoelectric element or a piezoresistive element as a pressure transducer. Piezoelectric crystals or piezoresistive pressure transducers mounted at or near the distal tips of pacing leads, for pacing applications, or catheters, for monitoring applications, are described in U.S. Pat. Nos. 4,023,562; 4,407,296; 4,432,372; 4,485,813; 4,858,615; 4,967,755; and 5,324,326; and PCT Publication No. WO 94/13200, for example. The desirable characteristics and applications for patient use of such leads or catheter bearing, indwelling pressure sensors are described in these and other patents and the literature in the field.
Other semiconductor sensors employ complimentary metal oxide semiconductor (CMOS) integrated circuit (IC) technology in the fabrication of pressure responsive silicon diaphragm bearing capacitive plates that are spaced from stationary plates. A change in capacitance due to pressure waves acting upon the diaphragm is measured, typically through use of a bridge circuit, as disclosed, for example, in the article "A Design of Capacitive Pressure Transducer" by Ko et al., in IEEE Proc. Symp. Biosensors, 1984, p.32. Fabrication for long term implantation and stability is, however, complicated.
In addition, differential capacitive plate, fluid filled pressure transducers employing thin metal or ceramic diaphragms have also been proposed for large scale industrial process control applications as disclosed, for example, in the article "A ceramic differential-pressure transducer" by Graeger et al., Philips Tech. Rev., 43:4:8693, Feb. 1987. The large scale of such pressure transducers does not lend itself to miniaturization for chronic implantation.
Improved capacitive pressure sensor implementations are disclosed in U.S. Pat. Nos. 5,564,434 and 5,535,752 to Halperin, both of which are incorporated herein by reference in their respective entireties. The '434 and '752 patents disclose a capacitive pressure and temperature sensing system for providing signals representative of absolute pressure and temperature of a body fluid. A sensor module includes a pickoff capacitor and a reference capacitor. The pickoff capacitor changes in capacitance in response to fluid pressure changes, while the reference capacitor is relatively insensitive to fluid pressure changes.
A constant current source provides for charging and discharging of the pickoff and reference capacitors. A pulse generator generates pickoff and reference timing pulses separating pressure related and temperature related charge time intervals of the pickoff and reference capacitors which vary as a function of the charge current and capacitance changes of the pickoff capacitor. Although the capacitive pressure sensing systems disclosed in the '434 and '752 patents provide for accurate sensing of body fluid pressure, a pressure sensing approach having increased sensitivity may be desirable in certain applications.
Despite the considerable effort that has been expended in designing such pressure sensors, a need continues to exist for a body implantable pressure sensor for accurately sensing the pressure of a body fluid which is not subject to mechanical limitations of conventional capacitive transducers. There continues to exist a need for a pressure sensing system that provides for improved sensitivity beyond that provided by a conventional mechanical pressure-to-capacitance transducer. The present invention fulfills these and other needs.